In recent years, as a wheel bearing device (drive wheel bearing device), there has been known a wheel bearing device including a hub wheel, a double row rolling bearing, and a constant velocity universal joint that are provided as a unit. That is, as illustrated in FIG. 11, this wheel bearing device includes a hub wheel 102 having a flange 101 extending in a radially outer direction, a constant velocity universal joint 104 having an outer joint member 103 fixed to the hub wheel 102, and a rolling bearing 100 arranged on an outer circumferential side of the hub wheel 102.
The constant velocity universal joint 104 includes the outer joint member 103, an inner joint member 108 arranged in a bowl-like section (cup section) 107 of the outer joint member 103, balls 109 arranged between the inner joint member 108 and the outer joint member 103, and a cage 110 for holding the balls 109. Further, a spline portion 111 is formed in an inner circumferential surface of a center hole of the inner joint member 108, and a spline portion provided at an end portion of a shaft (not shown) is inserted into the center hole so that the spline portion 111 on the inner joint member 108 side and the spline portion on the shaft side engage with each other.
Further, the hub wheel 102 includes a cylinder section 113 and the flange 101. A pilot section 115 having a shape of a short cylinder is provided in a projecting manner on an outer end surface 114 of the flange 101 (end surface located opposite to the joint), and a wheel (not shown) and a brake rotor (not shown) are mounted to the pilot section 115.
Further, a cutout portion 116 is provided in an outer circumferential surface of an end portion of the cylinder section 113 on the cup section 107 side, and an inner race 117 is fitted to the cutout portion 116 so as to form an inner member of the rolling bearing 100. A first inner raceway surface 118 is provided in the outer circumferential surface of the cylinder section 113 of the hub wheel 102 at a position in the vicinity of the flange, and a second inner raceway surface 119 is provided in an outer circumferential surface of the inner race 117. Note that, bolt insertion holes 112 are provided in the flange 101 of the hub wheel 102, and hub bolts are inserted into the bolt insertion holes 112, respectively, so as to fix the wheel and the brake rotor to the flange 101.
An outer member 105 of the rolling bearing 100 has double row outer raceway surfaces 120 and 121 provided on an inner circumference thereof. The first outer raceway surface 120 of the outer member 105 is opposed to the first inner raceway surface 118 of the hub wheel 102, and the second outer raceway surface 121 of the outer member 105 is opposed to the raceway surface 119 of the inner race 117. Rolling elements 122 are interposed between those opposing surfaces.
A partition wall 124 is provided on the cylinder section 113 of the hub wheel 102, and a coupling bolt member 125 is inserted into a through hole 124a of the partition wall 124. Further, a screw hole 127 is provided in a bottom wall 126 of the cup section 107, and the bolt member 125 is threadedly inserted into the screw hole 127.
Conventionally, there is known a torque transmitting structure formed of a face spline structure, in which a face spline 128 is provided in a surface of the bottom wall 126 of the cup section 107 that is opposed to the hub wheel (back surface), and a face spline 129 is provided in a surface of the cylinder section 113 of the hub wheel 102 that is opposed to the bowl-like section (Patent Literature 1). In this case, in each of the face splines 128 and 129, a plurality of projecting threads extending in a radial direction and a plurality of depressed threads extending in the radial direction are alternately arranged along a circumferential direction.
In the face spline structure as described above, there are attained such advantages that torque can be transmitted without a backlash, that stick-slip noise can be suppressed between the surface opposed to the hub wheel and the surface opposed to the bowl-like section, that the easiness of assembly is excellent, and that the light-weighting can be achieved.
By the way, to form the face spline as described above, there is known an apparatus that includes a punch having a tooth profile (tooth portion) formed therein for performing plastic working of the face spline, and a rockshaft having a center axis inclined at a predetermined angle, and is configured to form the face spline due to plastic deformation caused by rocking motion of the rockshaft (Patent Literature 2).